1. Field of the Invention
This invention relates to a method of producing a semiconductor substrate through a lamination process, and more particularly to a method of producing a semiconductor substrate through a lamination process at a state that a thickness of a buried oxide film is thin.
2. Description of Related Art
Recently, semiconductor substrates having a SOI structure that silicon layer or so-called SOI layer is formed on an oxide film are applied as a wafer for a high-performance LSI in electron devices because they are adaptable for speeding up the device and are low in the power consumption and excellent in the pressure resistance, resistance to environment and the like.
As the production method of the semiconductor substrate, there are known a so-called SIMOX method wherein a silicon wafer is implanted with oxygen ions at a high concentration and then subjected to a heat treatment at a high temperature to form an oxide film therein, and a method called as a so-called lamination process. In the lamination process, an oxide film is formed on at least one of a wafer for an active layer forming SOI layer and a wafer for a support substrate and both the wafers are laminated to each other through the oxide film and thereafter the wafer for the active layer is thinned to produce a semiconductor substrate in which SOI layer is formed on the buried oxide film as an insulating film.
Further, the lamination process can be classified into a grind polishing method, a PACE (Plasma Assisted Chemical Etching) method, an ion implantation exfoliation method (which is called as Smart Cut (registered trademark) method), an ELTRAN method and so on. Among them, the ion implantation exfoliation method is frequently used because the crystallinity of the active layer and the thickness uniformity of the active layer are good and the good surface flatness is obtained.
The production procedure of the semiconductor substrate through the lamination process is shown in FIG. 1. That is, a wafer 1 for an active layer and a wafer 2 for a support substrate are previously provided (step (a)), and an oxide film 3 is formed on at least one of both the wafers (the wafer 1 for the active layer in the illustrated embodiment) (step (b)), and then hydrogen ions (or inert gas ions) are implanted into the wafer 1 for the active layer to form an ion implanted layer 4 in the interior of the wafer 1 for the active layer (step (c)). Thereafter, the ion implanted face of the wafer 1 for the active layer is laminated onto the wafer 2 for the support substrate through the oxide film 3 (step (d)), and subjected to an exfoliation heat treatment to partly exfoliate the wafer 1 for the active layer at the ion implanted layer as a cleavage face (exfoliation face) (step (e)), and thereafter re-oxidation treatment is conducted for removing a damaged layer formed on the surface of the active layer (step (f)), and then the planarization treatment is conducted through a step (g) of removing the resulting oxide film to produce a semiconductor substrate 7 in which a silicon layer 6 is formed on a buried oxide film 5.
Lately, it is required to produce SOI wafers having a higher quality in association with the high integration of the semiconductor devices. For this end, it is increasingly demanded to make the thickness of the buried oxide film thinner, for example, up to a thickness of about 20 nm, or to directly laminate the silicon wafers to each other without utilizing the oxide film as to the laminated wafer.
When the laminated wafer is produced by thinning the buried oxide film or without forming the oxide film through the ion implantation exfoliation process, the wafer for the active layer and the wafer for the support substrate are laminated to each other by thinning the oxide film to be formed on either of the wafers or without forming the oxide film.
In the production of the laminated wafer having a thin oxide film inclusive of the case that the buried oxide film is not formed, however, when the exfoliation heat treatment is carried out after the lamination, blisters are generated between the wafer for the support substrate and the oxide film, or there are generated voids extending from the oxide film to the active layer.
That is, in the conventional production of the semiconductor substrate through the lamination process, defects such as blisters or voids may be caused at the laminated interface. Particularly, such blister or void defects tend to frequently occur as the thickness of the buried oxide film existing between two semiconductor wafers becomes thinner, which comes into a serious problem in the production of the laminated semiconductor substrate having a thin oxide film or having no oxide film.
As a countermeasure on the frequent occurrence of blisters or voids when the thickness of the buried oxide film existing between the two semiconductor wafers is made thinner, JP-A-2004-259970 proposes that the thickness of the wafer for the active layer is increased to increase the thickness of the active layer to thereby raise the hardness of the active layer.
However, even when the thickness of the active layer is made thick, if the thickness of the buried oxide film is thin, the blisters or voids occur naturally. Also, when the thinning of the active layer is promoted, the thickness of the active layer is thickened at the midway step for the purpose of raising the hardness, which takes a great labor in the subsequent thinning treatment and causes the deterioration of the quality. That is, when the thickness of the active layer at the midway step is thick, it is required to conduct the thinning of the active layer by a treatment of thermal oxidation+removal of oxide film, or by grinding or polishing treatment for obtaining a final desired thickness of the active layer. In the latter case, as the treating quantity (oxidation quantity, etching quantity, grinding or polishing quantity) increases, the thickness uniformity of the active layer is deteriorated.